Aviation Theory

(Continued from IAP: part 1)

IAP CHARTS

The IAP Charts provide a graphical presentation of holding procedures, if required, the approach procedure itself and the missed approach procedure. IAP charts are available for all airports with established instrument approach procedures which have been approved by the FAA. There are two publishers of charts in the United States, NOS (National Ocean Service) and Jeppesen. There are some differences in the two and which is "better" is mostly personal preference. Both will be presented here and the differences will be shown.

There is certain information provided on both types of charts which includes:
identification of the approach a plan view a profile view holding procedures radio facility information airport topographic information a runway layout.

The descriptions below refer to the NOS and Jeppesen chart examples. The section discussed is marked on the chart by the corresponding letter.

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IDENTIFICATION OF THE APPROACH - (A)

The identification is at the top of the chart and generally includes the name of the airport, an abbreviation of the type of approach, the runway served, and additional information necessary to distinguish it from other similar approaches at the same airport.

Some information about the type approach can be learned just from the name. For example an approach named ILS DME Rwy 27 indicates both ILS and DME are required. ILS Rwy 27 would indicate that DME is not required. If a letter follows the navaid, for example VOR-A, it indicates that only circling minimums are published and no straight-in approach is available. This could be because of of alignment problems or steep descent requirements.

RADIO FACILITY INFORMATION - (B)

The communications frequencies are listed near the top of the chart in the order they are usually needed.

PLAN VIEW - (C)

The plan view is a conformal projection which shows the correct angular relationships of the navaids and fixes in relation to each other. Horizontal distances are measured in nautical miles to facilitate the use of DME which reads nm.

The airport and its immediate environment is also shown on the plan view. Although obstacle clearance is provided by the approach, certain obstacles may be indicated on the plan view and may be handy for visual references in some situations.

The position of each navaid, with frequencies, ident and course in degrees magnetic is also included in the plan view. The main procedure course is shown as a heavy line with a directional arrow. An required procedure turn is also indicated.

The missed approach is shown as a dashed line with a directional arrow. Although holding patterns are not usually strictly part of the approach, they are shown with a magnetic direction and a directional arrow.

MINIMUM SAFE ALTITUDE - (D)

The Minimum Safe Altitude (MSA) circle is found in the plan view of NOS charts and in the heading section of Jeppesen charts. The MSA circle, which may contain several sectors, indicates the minimum altitude that provides 1000 ft obstacle clearance within 25 nm, of the radio facility at the center of the MSA circle. It should be noted that MSAs do not guarantee navigation or communication reception and should be considered emergency minimum altitudes for IFR procedures.

PROFILE VIEW - (E)

The profile view is directly below the plan view and also uses a bold line with directional arrows to depict the main approach course. The angles shown in the profile view are generally not to scale. The most common angle for an ILS approach is 3°. Procedure turns are indicated by a horizontal line with altitudes, distance and time requirements listed close by.

Radio facilities, such as the OM, LOM, MM, are also shown in the profile. The missed approach procedure is again represented with a dotted line and directional arrow.

Vertical distances in the profile view are shown as feet MSL (Mean Sea Level). Airport elevation (in feet MSL) is shown in the profile and is the highest point in the landing area. On NOS charts it is shown as Elev and on Jeppesen charts it is shown as Apt. Elev at the top right and as Apt. near the runway in the profile diagram. Height Above Airport (HAA) is used when specifying circling minimums and alternate minimums.

Touchdown Zone Elevation (TDZE) is the highest elevation in the first 3000 ft of the runway and is shown near the runway on the IAP chart when straight-in minimums are authorized. Height Above Touchdown zone (HAT) is also included when straight-in minimums are published.

Threshold Crossing Height (TCH) is the theoretical height of the aircraft's glideslope antenna above the runway threshold if the aircraft is on the glideslope. The TCH is NOT the height of the wheels above the threshold.

APPROACH MINIMUMS - (F)

Approach minimums are based on the category of aircraft, equipment carried and pilot qualifications. The performance categories of the aircraft are based on maneuverability, which is largely dependent on airspeed. The categories are based on an airspeed of 1.3 x Vso, where Vso is the stall speed in the landing configuration at maximum landing weight. The categories and speeds are listed in the following table:

Once you are visual, the landing minimum is determined by visibility. Visibility may be Runway Visual Range (RVR) in feet or visibility in statute miles.

Minimums may be increased due to a number of variables including, higher speed than normal category and inoperative components, such as MM not available.

Some nonprecision approaches have a visual Descent Point (VDP) from which descent from the normal MDA may be commenced, provided the runway environment is clearly visible.

Another variable which may be encountered on some approaches is a sidestep maneuver, where an ILS approach is flown to one runway and landing is on another parallel runway within 1200 ft laterally. The minimums for a sidestep may be higher than the straight-in minimums, but, are generally lower than circle to land minimums.

TIMING TO THE MISSED APPROACH POINT - (G)

A small table is included for nonprecision approaches which shows the time from the FAF to the MAP. This table is also included on ILS approaches and is for use when the glideslope is not functioning and the approach is flown as a localizer approach.

The pilot should always start timing at the FAF, even on an ILS approach. If the glideslope fails, you could continue with a nonprecision localizer approach and be able to identify the MAP.

This has been an overview of the IAPs and the actual procedures and techniques of flying the approach will be covered in future lessons.

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